1. Field of the Invention
This invention pertains to an oil recovery method, and more specifically to a method for recovering oil or petroleum from a subterranean, viscous petroleum-containing, unconsolidated formation such as a tar sand deposit.
2. Description of the Prior Art
There are known to exist throughout the world many subterranean petroleum containing formations from which the petroleum cannot be recovered by conventional means because of the relatively high viscosity thereof. The best known of such viscous petroleum containing formations are the so-called tar sands or bituminous sand deposits. The largest and most famous such deposit is in the Athabasca area in the northeastern part of the province of Alberta, Canada, which is known to contain over 700 billion barrels of petroleum. Other extensive deposits are known to exist in the western part of the United States, and Venezuela, and lesser deposits in Europe and Asia.
Tar sands are frequently defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The hydrocarbons contained in tar sand deposits are usually highly bituminous in character. The tar sand deposits are generally arranged as follows. Fine quartz sand is coated with a layer of water and the bituminous material occupies most of the void space around the wetted sand grains. The balance of the void volume may be filled with connate water, and occasionally a small volume of gas which is usually air or methane. The sand grains are packed to a void volume of about 35%, which corresponds to about 83% by weight sand. The balance of the material is bitumen and water. The sum of bitumen and water will almost always equal about 17% by weight, with the bitumen portion varying from around 2% to around 16%.
It is an unusual characteristic of tar sand deposits that the sand grains are not in any sense consolidated, that is to say the sand is essentially suspended in the solid or nearly solid hydrocarbon material. The API gravity of the bitumen usually ranges from about 6 to about 8, and the specific gravity at 60.degree. F. is from about 1.006 to about 1.027. Approximately 50% of the bitumen is distillable without cracking, and the sulfur content may be as high as between 4 and 5% by weight. The bitumen is also very viscous, and so even if it is recoverable by an in situ separation technique, some on-site refining of the produced petroleum must be undertaken in order to convert it to a pumpable fluid.
Bitumen may be recovered from tar sand deposits by mining or by in situ processes. Most of the recovery to date has been by means of mining, although this is limited to instances where the ratio of the overburden thickness to tar sand deposit thickness is economically suitable, generally defined as one or less. In situ processes have been proposed which may be categorized as thermal, such as fire flooding or steam injection, and steam plus emulsification drive processes. Generation of the heat necessary to mobilize the bitumen by means of a subterranean atomic explosion has been seriously considered.
The known processes most clearly resembling that of the present invention are disclosed in U.S. Pat. Nos. 2,412,765; 3,608,638 and 3,838,738. The '765 patent discloses a process in which low boiling solvents such as propane and butane are employed to recover oil from partially depleted formations. In this process the oil recovery is made as complete as possible and the formation is a consolidated or self-supporting formation. In the process of the '638 patent a hydrocarbon solvent such as benzene, platformate or kerosene, at a temperature in the range of 300.degree.-700.degree. F., is injected into the top of tar sands at an injection well and forced through the formation to an adjacent production well. Injection of the solvent and production of oil are continued to maintain a gaseous phase across the top of the formation. The tar sand oil is made more mobile as a result of heating and dissolution of the solvent into the oil, whereby the oil drains into the production well and is lifted to the surface. A requirement of solvents to be used in the latter process is that the solvent is miscible in the tar sand oil without precipitating constituents in the oil. Preferred solvents are those which have good solvent properties such as aromatic hydrocarbons or mixtures of hydrocarbons containing substantial amounts of aromatic hydrocarbons. Highly parafinic hydrocarbons such as liquified petroleum gases are said not to be suitable because of their inability to dissolve the asphaltic constituents of the tar sand oil. The '738 patent is directed to a process in which petroleum is recovered from tar sand deposits by first creating a fluid communication path low in the formation. This is followed by injecting a heated fluid, aqueous or non-aqueous, into the fluid communication path. This in turn is followed by injecting a volatile solvent such as carbon disulfide, benzene or toluene into the preheated flow path and continuing to inject the heating fluid. The more volatile solvent is vaporized and moves upward into the formation where it dissolves petroleum, loses heat and condenses thereafter, flowing down, carrying dissolved bitumen with it into the preheated flow path. The low boiling point solvent effectively cycles or refluxes within the formation and is not produced to the surface of the earth. Bitumen is transferred from the volatile solvent to the heating fluid, continually passing through the communication path, and bitumen and heating fluid are recovered together as a mixture or solution.
U.S. Pat. No. 2,862,558 is less closely related to the present invention but is of interest in that viscous petroleum is recovered by this process from consolidated sandstone formations, such as the Barton tar sands, by the injection of steam and solvent vapor -- kerosene vapor, for example -- with what is described as improved results over the abovementioned '765 patent.
It has not been recognized in preceding patents or elsewhere in the prior art that complete removal of the hydrocarbons or bitumen from a tar sand is undesirable. When the tar content is completely removed, the remaining sand particles are not connected to each other and effectively become free-flowing. This tends to result in formation subsidence and in blockage of the pump intake and/or production line in the well by sand bridging.
Despite the many proposed methods for recovering bitumen from tar sand deposits, there has still been no successful exploitation of such deposits by in situ processing on a commercial scale up to the present time. Accordingly, in view of the lack of commercial success of any of the methods proposed to date and especially in view of the enormous reserves present in this form which are needed to help satisfy present energy needs, there is a substantial need for a satisfactory method for recovery of bitumen from tar sand deposits.